Many modern electronic circuits employ electronic filters to remove unwanted signal components. One conventional filter structure may include an inductor and a capacitor (an LC) circuit. FIG. 1A illustrates a conventional LC circuit. FIG. 1B illustrates an equivalent balanced differential version of the LC circuit. Electronic filters are used in a large number of applications, including bi-directional communications systems, the various types of Digital Subscriber Line (xDSL), and other digital carrier systems. xDSL may include, for example, asymmetric digital subscriber line (ADSL), high-speed digital subscriber line (HDSL) and very high-speed digital subscriber line (VDSL) systems.
In general, electronic filters see at least two sets of frequencies in a received signal. One range of frequencies is allowed to pass through the filter without being attenuated and is referred to as the passband. A second range, which is filtered out and not allowed to pass through to the filter output, is referred to as the stopband. When driving a resistive load (Z0), most conventional electronic filters, including those illustrated in FIGS. 1A and 1B, have the same nominal, resistive input impedance (Z0) in the filter passband. Conversely, in the stopband, the input impedance becomes more purely reactive. In other words, the input voltage of the filter will vary as the frequency of the applied signal varies. This significantly impairs bi-directional communications. Additionally, if conventional electronic filters having different filter responses are coupled together in series, the filters may interact in an undesirable fashion.